1. Field of the Invention
The present invention relates to a system for controllably measuring a formation resistivity using induction chokes to form electrically isolated well casing sections such that the well casing can be used as the formation contact electrodes. In one aspect, it relates to a petroleum production well and a method of operating the well while also being able to measure the formation resistivity using the well casing as the formation contact electrodes.
2. Description of Related Art
Formation resistivity is a fundamental measurement for the analysis and characterization of possible hydrocarbon production zones that the well passes through. Such a measurement is informative because the measured resistivity reflects both the porosity of the formation and the composition of the fluids that occupy the pore spaces. See e.g., WELL LOGGING FOR EARTH SCIENTISTS, Darwin V. Ellis, Elsevier, New York, USA, 1987, ISBN 0-444-01180-3 (incorporated by reference for background).
Measuring formation resistivity is customarily only done as part of the wireline well logging procedure before the well casing has been installed (open-hole) because a conventional metallic casing will normally act as an electrical short-circuit between elements of the formation, preventing the formation resistivity from being measured.
As a producing well withdraws fluids from a formation, it causes fluid migration toward the producing zones from more distant parts of the reservoir. The moving fluids may be oil, gas, water (usually as brine), or a mixture of these. Because hydrocarbons are electrically non-conductive and brine is relatively conductive, it would be valuable to measure resistivity changes in a producing well as an aid in monitoring changes in the spatial distribution of fluid migrations that are being caused by production and characterizing the changing conditions of the reservoir. However, because the resistivity normally cannot be measured after the well has been completed (i.e., after the well casing is installed), this analytical method is not generally or readily available to assist in planning of the production process and managing the reservoir during production operations.
All references cited herein are incorporated by reference to the maximum extent allowable by law. To the extent a reference may not be fully incorporated herein, it is incorporated by reference for background purposes, and indicative of the knowledge of one of ordinary skill in the art.
The problems and needs outlined above are largely solved and met by the present invention. The casing is fitted with a number of electrically inductive chokes that are placed on the casing sections at the time the casing is set in the well. By suitable design these induction chokes act as impedances to time-varying current flow (e.g., alternating current) along the casing, and thus act to divert the current from the casing into the formation. Because the current must then pass through the formation, the impedance to such current flow provides a method to measure the resistivity of the formation adjacent to the borehole section where the induction chokes are placed and it allows the isolated casing sections to act as separate electrodes.
A variety of choke dispositions may be employed to control the path of current flow outside the casing and thus enable formation resistivity to be measured at various distances from the casing or various depths into the formation. The principle is similar to the conventional open hole focused wireline electric logging tools known as xe2x80x9claterologs.xe2x80x9d In conventional open hole laterologs, an array of current electrodes contact the exposed formation surface within the borehole, and the electrical potentials on these electrodes are controlled in a manner that causes a focused current to flow into the formation. Casing mounted chokes provide a similar function, but through the casing (for wellbores having a well casing or liner) and by using the casing sections as electrodes.
By controlling the frequency of the time-varying or alternating current, the impedance presented by the induction chokes may be altered, which allows for a separate and independent method of controlling the measurement conditions. Furthermore, one of the distinct advantages of the present invention is that the formation resistivity measurements may be performed during petroleum production operations, without changing the production well configuration (i.e., removing the production tubing) and without interrupting production processes.
In accordance with one aspect of the present invention, a system for measuring a formation resistivity in a petroleum well is provided. The system comprises a first induction choke, a second induction choke, and a device. The first induction choke is located about a piping structure of the well. The second induction choke is also located about the piping structure of the well, but the first induction choke is distally spaced from the second induction choke. The device is located outside of the piping structure and comprises two terminals. A first of the terminals extending from the device is electrically connected to the piping structure on one side of the first induction choke. A second of the terminals extending from the device is electrically connected to the piping structure on another side of the first induction choke between the first and second induction chokes, such that the downhole device is electrically connected across an outside of the first induction choke. The system can further comprise other induction chokes, other terminals extending from the device, a current sensor, a surface power source, a power transformer, a communications transformer, a surface modem, a downhole modem, a direct current power supply, and/or a power amplifier. The piping structure can comprise at least a portion of a well casing, such that the well casing acts as an electrode for making formation resistivity measurements due to the induction chokes.
In accordance with another aspect of the present invention, a petroleum well for producing petroleum products is provided. The petroleum well comprises a piping structure, a first induction choke, a second induction choke, a third induction choke, and a downhole device. The piping structure of the well extends within a formation, which may comprise an oil or gas production zone. The first induction choke is located downhole about the piping structure. The second induction choke is also located downhole about the piping structure, but the second induction choke is distally spaced from the first induction choke. In addition, the third induction choke is located downhole about the piping structure, and the third induction choke is distally spaced from the first and second induction chokes. The downhole device comprises four terminals. A first device terminal is electrically connected to the piping structure on a one side of the first induction choke. A second device terminal is electrically connected to the piping structure on another side of the first induction choke between the first and second induction chokes, such that the downhole device is electrically connected across an outside of the first induction choke. A first electrode terminal is electrically connected to the piping structure between the second and third induction chokes. A second electrode terminal is electrically connected to the piping structure such that the third induction choke is located between the electrical connection location on the piping structure of the first and second electrode terminals.
In accordance with yet another aspect of the present invention, a petroleum well for producing petroleum products is provided. The petroleum well comprises a well casing, a power source, a surface modem, a first induction choke, a second induction choke, a third induction choke, and a downhole device. The well casing extends into a formation. The power source is adapted to output a time-varying current. The power source has two power source terminals. A first of the power source terminals is electrically connected to the casing at the surface. A second of the power source terminals is electrically connected to the formation at the surface. The surface modem has two surface modem terminals. A first of the surface modem terminals is electrically connected to the casing at the surface. A second of the surface modem terminals is electrically connected to the formation at the surface. The first induction choke is located downhole about the casing. The second induction choke is located downhole about the casing, wherein the second induction choke is distally spaced from the first induction choke, and wherein the second induction choke is located farther downhole than the first induction choke. The third induction choke is located downhole about the casing, wherein the third induction choke is distally spaced from the first and second induction chokes, and wherein the third induction choke is located farther downhole than the second induction choke. The downhole device comprises four terminals, a downhole modem, and a current sensor. A first device terminal is electrically connected to the casing on a source-side of the first induction choke. A second device terminal is electrically connected to the casing on another side of the first induction choke between the first and second induction chokes, such that the downhole device is electrically connected across an outside of the first induction choke. A first electrode terminal is electrically connected to the casing between the second and third induction chokes. A second electrode terminal is electrically connected to the casing such that the third induction choke is located between the electrical connection location on the casing of the first and second electrode terminals. The downhole modem is communicably coupled to the device terminals, such that the downhole modem can send and receive communication signals along the casing via the device terminals. The current sensor is adapted to measure current flowing through the first electrode terminal and/or the second electrode terminal. The current sensor is communicably coupled to the downhole modem, such that the downhole modem is adapted to receive measurement data from the current sensor and transmit the measurement data to the surface modem via the casing.
In accordance with still another aspect of the present invention, a method of producing petroleum products from a petroleum well is provided. The method comprises the following steps (the order of which may vary): (i) providing a piping structure of the well; (ii) providing a system for measuring formation resistivity in the well, the system comprising: (a) a first induction choke located about the piping structure, (b) a second induction choke located about the piping structure, wherein the first induction choke is distally spaced from the second induction choke, and (c) a device located outside of the piping structure and comprising two terminals, a first of the terminals extending from the device being electrically connected to the piping structure on one side of the first induction choke, and a second of the terminals extending from the device being electrically connected to the piping structure on another side of the first induction choke between the first and second induction chokes, such that the downhole device is electrically connected across an outside of the first induction choke; and (iii) measuring the resistivity of a formation with the system while producing petroleum products with the well. The method may further comprise the step of: (iv) monitoring for changes in formation resistivity while producing petroleum products with the well by repeating the measuring step as needed. Also, the method may further comprise the steps of: (iv) providing a surface modem having two surface modem terminals, a first of the surface modem terminals being electrically connected to the piping structure at the surface, and a second of the surface modem terminals being electrically connected to the formation at the surface; (v) providing a downhole modem for the system, the downhole modem being communicably coupled to the terminals; and (vi) transmitting formation resistivity data generated in the measuring step to the surface modem with the downhole modem via the piping structure. In addition, the method may further comprise the steps of: (iv) providing a third induction choke as a part of the system, the third induction choke being located about the piping structure, and being distally spaced from the first and second induction chokes; (v) providing a measuring electrode terminal extending from the device and that is electrically connected to the piping structure between the second and third induction chokes; (vi) providing an electrical return electrode terminal extending from the device and that is electrically connected to the piping structure such that the third induction choke is located between the electrical connection location on the piping structure of the measuring electrode terminal and the electrical return electrode terminal; and (vii) providing a current sensor on the measuring electrode terminal such that the current sensor can measure electrical current flow within the measuring electrode terminal, such that the electrical current flow in the measuring electrode terminal correlates to the formation resistivity.